1. Field of the Invention
This invention relates to a method and apparatus for consolidating particulate material, such as powders, and more particularly, to a system and method for consolidating particulate material by applying relatively long duration current flow at relatively low current densities to the particulate material in order to achieve densities in excess of ninety percent (90%) of the theoretical maximum density for the particulate material.
2. Description of Related art
The consolidation of particulate material under relatively high compaction pressure using molds and dies to manufacture parts has become a frequently used industrial process. One of the major limitations of the powder material compaction process is that, with most materials, less than full densification is achieved during the compaction process. Typically, powder material consolidation results in less than ninety-three percent (93%) of its full theoretical density for many powders and for difficult to compact materials (such as stainless steel) less than eighty-five percent (85%) of theoretical density is achieved. Less than full density, results in degraded material properties, such as strength, stiffness, magnetisity and the like. High density is required to enable particulate material consolidation to make higher performance parts, such as gears, for example, for use in automobiles because high strength is often required.
U.S. Pat. Nos. 4,929,415; 4,975,412; 5,084,088; 5,529,746; 5,380,473 are examples of consolidation techniques of the type used in the past. For example, Okazaki discloses a method for sintering and forming powder. This method uses a high voltage of 3 KV or more which is applied to a mold filled with the powder using an electrode which maintains a high current of 50 KAcm−2 or greater for a period of time from 10 to 500 microseconds.
Similarly, U.S. Pat. No. 4,975,412 also discloses a method of processing superconducting materials which utilizes, again, a high voltage and current density to provide sharp bonding between or among the particulate material.
Still another example is U.S. Pat. No. 5,529,746 issued to Knoss which discloses processing the powders using one to three electric current pulses from 5×10−5 to 5×10−2 second duration and high electric power applied to the punches of the press.
Thus, the typical technique for consolidating the particulate material is to use a relatively high current pulse of fairly short duration to cause consolidation of the powder. A problem with this approach has been that under these conditions electrical arcing may occur at the interface between the powder and the current-conducting punches. This arcing will severely limit the useful life of the punches and, therefore, must be overcome in order to make this technique commercially viable.
Still another problem of the prior art is that the walls of the molds or dies used during the consolidation process required an insulator, such as ceramic. One significant problem with this approach is that the ceramic used for insulating the walls were not suitable for generating parts having shapes which require intricate details because when the intricate details are machined into the ceramic insulators and the insulators placed in the die, the ceramic would sometimes crack or chip upon use during the consolidation process.
Another problem with prior art techniques is that they did not permit tailoring of the power input to the powder mass to allow controlled power input. This resulted in inconsistent densification of parts manufactured using the consolidation process.
What is needed, therefore, is a system and method for consolidating powders which will avoid the problems encountered by the techniques used in the past.